Since the pattern of horizontal disparities created by a frontal surface depends on the distance of the surface from the observer, additional information about distance is needed in order to judge whether a surface lies in a frontal plane. Rogers and Bradshaw (1995 Perception 24 155 – 179) showed that both vertical disparities and vergence angle can be used to scale the curvature of surfaces in a horizontal direction. In the present experiments, we measured the extent of frontal plane scaling as a function of the location and eccentricity of the vertical disparity information. Observers were presented with a series of random-textured stereoscopic surfaces with vertical disparity information appropriate for surfaces located at distances between 28 cm and infinity. The observer's task was to vary the pattern of horizontal disparities until the surface appeared to lie in a frontal plane. The stereoscopic images were masked to reveal either a circular, or an annular, or a rectangular patch with dimensions between 10 deg and 70 deg. Maximum scaling was found for the 70 deg diameter circular patch; it decreased by 50% when the patch was masked down to 20 deg in diameter. Scaling remained at over 80% when the central 60,deg of the display were masked. Scaling was reduced more when the horizontal width of the rectangle was made smaller than when the vertical height was made smaller. Vertical disparities are most effective for frontal plane scaling when they are present in more eccentric regions of the visual field, especially in a horizontal direction.
